Kinetic Study of the Phenolysis of O-Methyl and O-Phenyl O-2,4-Dinitrophenyl Thiocarbonates and O-Ethyl 2,4-Dinitrophenyl Dithiocarbonate

Abstract

The reactions of a series of phenols with O-methyl O-2,4-dinitrophenyl thiocarbonate (MDNPTOC), O-phenyl O-2,4-dinitrophenyl thiocarbonate (PDNPTOC), and O-ethyl 2,4-dinitrophenyl dithiocarbonate (EDNPDTC) are studied kinetically in water, at 25.0 degrees C and an ionic strength of 0.2 M (KCl). All reactions show pseudo-first-order kinetics under an excess of phenol over the substrate, and are first order in phenoxide anion. The reactions of EDNPDTC show a linear Brønsted-type plot of slope beta = 0.67, suggesting a concerted mechanism. On the other hand, the phenolyses of MDNPTOC and PDNPTOC exhibit linear Brønsted-type plots of slopes beta = 0.27 and 0.28, respectively, consistent with stepwise mechanisms where the formation of an anionic tetrahedral intermediate (T(-)) is rate determining. By comparison of the kinetics and mechanisms of the reactions under investigation with similar reactions, the following conclusions arise: (i). Substitution of S(-) by O(-) in the intermediate T(-) destabilizes this species. (ii). The change of DNPO in T(-) to DNPS also destabilizes this intermediate. (iii). Substitution of MeO by PhO as the nonleaving group of the substrate does not affect the kinetics, probably by a compensation of electronic and steric effects. (iv). The change of an amino group in a tetrahedral intermediate to a phenoxy group destabilizes the intermediate.

Phenolysis of diaryl thiolcarbonates and thionocarbonates

“…A comparison of the nucleophilic rate constants (k N ) for the phenolysis of thiolcarbonates 1 and 2 in 44 wt% ethanol-water (this work) with those of the same reactions of phenyl and 4-chlorophenyl 4-nitrophenyl carbonates in water [10,12] shows that the carbonates are more reactive (about 80-100-fold) than the corresponding thiolcarbonates towards a given phenoxide. This is in accordance with the larger k N values found in the following reactions when a benzenethiolate leaving group is substituted by its phenoxide analogue: (i) the phenolysis of alkyl aryl carbonates [9] compared with that of alkyl aryl thiolcarbonates [13] (10-40-fold); (ii) the phenolysis of methyl 2,4-dinitrophenyl thionocarbonate, [14] relative to that of O-ethyl 2,4-dinitrophenyl dithiocarbonate; [14] (iii) the benzenethiolysis of methyl aryl carbonates [19] compared with that of ethyl S-aryl thiolcarbonates [19] (2-5-fold). Also, the reactions of secondary alicyclic amines with methyl 2,4,6-trinitrophenyl carbonate [21] show greater k N values than those with ethyl S-(2,4,6-trinitrophenyl) thiolcarbonate.…”

“…These results indicate that the change of thiocarbonyl by carbonyl as the electrophilic group destabilizes the intermediate T À in such a way that the mechanism changes from stepwise to concerted. This is the same effect as that found for the phenolysis of 4-nitrophenyl and 2,4-dinitrophenyl methyl thionocarbonates (stepwise mechanisms) [11,14] and the corresponding carbonates (concerted processes). [9] Other examples are: (i) the phenolysis of bis(nitrophenyl) thionocarbonate is stepwise, [11] whereas that of bis(nitrophenyl) carbonate is concerted [11] and (ii) the phenolysis of phenyl 2,4-dinitrophenyl thionocarbonate is driven by a stepwise mechanism, [14] in contrast to that of phenyl 2,4-dinitrophenyl carbonate, which is concerted.…”

“…[11] The b 1 values are also in accordance with those found in the stepwise phenolyses of methyl and phenyl 2,4-dinitrophenyl thionocarbonates. [14] Effect of the non-leaving group Figure 2 shows that for the reactions of thionocarbonates 3-5 with the two most basic phenoxide anions (pK a of the corresponding phenols greater than 7.5), the formation of the anionic tetrahedral intermediate (T À ) is the rate-determining step. Therefore, for the reactions of these nucleophiles, k N ¼ k 1 .…”

“…There have been many reports on the kinetics and mechanisms of the phenolysis of esters [1][2][3][4][5][6] and thioesters, [7,8] but less regarding the phenolysis of methyl aryl carbonates, [9] diaryl carbonates, [10][11][12] O-alkyl S-aryl thiolcarbonates [8,13] alkyl aryl thionocarbonates [8,11,14] and diaryl thionocarbonates. [8,11,14] The kinetic studies on the phenolysis of methyl aryl carbonates [9] and O-ethyl S-aryl thiolcarbonates [13] show concerted mechanisms, in contrast to those of methyl aryl thionocarbonates, which show stepwise mechanisms. [11,14] To our knowledge, there are only two works in the literature on the mechanism of the phenolysis of diaryl thionocarbonates (that of bis(4-nitrophenyl) thionocarbonate [11] and phenyl 2,4-dinitrophenyl thionocarbonate [14] ) and there are no reports on the phenolysis of O-aryl S-aryl thiolcarbonates.…”

“…[8,11,14] The kinetic studies on the phenolysis of methyl aryl carbonates [9] and O-ethyl S-aryl thiolcarbonates [13] show concerted mechanisms, in contrast to those of methyl aryl thionocarbonates, which show stepwise mechanisms. [11,14] To our knowledge, there are only two works in the literature on the mechanism of the phenolysis of diaryl thionocarbonates (that of bis(4-nitrophenyl) thionocarbonate [11] and phenyl 2,4-dinitrophenyl thionocarbonate [14] ) and there are no reports on the phenolysis of O-aryl S-aryl thiolcarbonates.…”